1. Field of the Invention
This invention relates to a method and apparatus for forming a heat-resistant synthetic resin container, and more particularly to such a method and apparatus in which a barrel and shoulder portions of a primary blow molded article is heated uniformly in a short time before secondary blow molding.
2. Description of the Related Art
Generally, a thin synthetic resin packing container called "a biaxially oriented blow container" is formed by placing in the mold an injection or extrusion molded preform of a suitable temperature for stretching and then by stretching the preform vertically or longitudinally while expanding the preform transversely with the pressure of gas blown into the preform.
However, the above-mentioned conventional container has a problem that the barrel portion of the container made of certain kinds of material would be deformed on filling up with the contents of high temperature.
Consequently a so-called oven blow molding method is proposed. In this conventional molding method, the blow molding step to be carried out after temperature control of the preform is divided into primary and secondary subdivided steps. In the primary blow molding, the article is formed so as to be longer than the final product to be obtained in the secondary blow molding. This primary blow molded article is then thermally shrunk by heat treatment, whereupon this article is secondary blow molded to form a final product. According to this conventional method, it is possible to obtain a heat-resistant container whose heat resistance is improved by heat treatment before the secondary blow molding.
FIG. 16 shows an article obtained by the oven blow molding. This oven blow molding method, as described in detail in, for example, Japanese Patent Laid-Open Publications Nos. HEI 3-205124 and HEI 3-234520, which are applicant's prior applications, comprises the step of obtaining a primary blow molded article 70 after temperature control of an injection or extrusion molded preform 60 having a suitable temperature for stretching, the step of obtaining a thermally shrunk article 80, and the step of obtaining a secondary blow molded product 90, the material of the preform 60 being polyethylene terephthalate (hereinafter abbreviated as PET), for example.
Specifically, the primary blow molded article 70 is obtained by stretching the preform 60 longitudinally with centering-guiding by a stretch rod (not shown) inserted into the preform 60 from its neck portion and, at the same time by expanding the longitudinally stretched preform 60 transversely by injecting pressurized fluid such as air into the preform 60.
The thermally shrunk article 80 is obtained by thermally shrinking the primary blow molded article 70 longitudinally and transversely by heating the article 70 in the atmosphere of 180.degree. C. to 280.degree. C. The thermally shrunk article 80 has a height substantially equal to or slightly longer than that of the final product, and a transverse size smaller than that of the final product.
The secondary blow molded product 90 is obtained by placing the thermally shrunk article 80 in the cavity of a secondary blow mold for molding the final product and then by introducing air into the thermally shrunk article 80 to be stretched transversely.
According to the oven blow molding method, during the heat treatment step to be carried out before the secondary blow molding, distortion created in the primary blow molding step is eliminated to increase the degree of crystallization so that the article will be resistant to severe temperature condition in the second blow molding step.
In order to obtain such a heat-resistant container, it is necessary to raise the temperature of the molded article to such an extent that the degree of crystallization can be improved.
However, in the conventional heating furnace, conventional heat transfer alone to the molded article in the atmosphere is insufficient for smooth raise of the temperature.
Since it would take a long time to get the temperature to a sufficiently level to obtain a certain degree of crystallization for adequate heat resistance of the article, it is required to lengthen the travelling path of the heated article or to protract the heating time, so that the molding apparatus including the travelling path for heating would be larger in size or the molding cycle would be longer.
In such heat treatment, it is also required to shrink the entire primary blow molded article uniformly to reduce the molding time during the secondary blow molding and to cause uniform heat resistance.
Practically on some occasions, however, the preform after temperature control would not retain a uniform circumferential temperature distribution. When the preform without a uniform circumferential temperature distribution undergoes the primary blow molding, the extent of circumferential stretch would vary locally to cause irregular wall thickness. Therefore, as the primary blow molded article having such non-uniform thickness is thermally treated, uniform thermal shrinking in the circumferential direction cannot be achieved. On some occasions, the circumferential shrinking would be irregular depending on the direction of hot air blow in the oven, and/or the coefficient of contraction would vary due to the difference in axial stretching magnification and wall thickness. When the thermally shrunk article is placed in the cavity and the mold is clamped for the second blow molding, the less shrunk portion would be sandwiched between the parting surfaces of the secondary blow mold and would be partly left as fins. Because of fins, the secondary blow molded article as the final product are used to be disposed of as a fault so that the yield of the heat-resistant container might be impaired.